Explant cultures of human and colon have the ability to enzymatically convert various classes of chemical carcinoges, such as polycyclic aromatic hydrocarbons, N-nitrosamines, mycotoxins and hydrazines, into metabolites which react with cellular macromolecules, such as DNA and protein. A 100-fold interindividual variation in the binding of benzo (a) pyrene (BP) to DNA was found. The carcinogen-DNA adducts for aflatoxin B1, BP, dimethylnitrosamine ald 1,2-dimethylhydrazine have been identified. Secondary bile acids significantly enhanced the binding of BP to DNA. This effect was mediated by increased uptake of BP into the cells and by modification of the metabolic pathway of BP.